A locking device for a mechanical jack of the above-mentioned kind is known in particular from French publication 2 652 129, but for manufacturers incorporating such seats in various vehicles it is desirable that these mechanisms be provided with an additional safety locking member useful in situations when, due to a false manipulation or lack of attention, the primary locking device of the respective jack is not perfectly engaged, the additional safety locking member automatically locking the jack by inertia when there is either a lateral impact or a front or rear impact.
It is necessary to have a safety device enabling an instantaneous locking of the rod of the jack with respect to the casing when there is an impact. Actually, it may happen, upon an abrupt braking of the respective vehicle, that the adjusting jack(s) of one of the seats are unlocked and therefore either the back portion or the seat portion of the seat can move with a projection, the speed of which projection being equal to the initial speed of the vehicle, thereby possibly causing a sever injury to the passenger sitting on the respective seat.
The present invention remedies these disadvantages by providing each mechanical jack not only with a conventional (primary) locking device but also with a safety locking member working by means of inertia.
Moreover, in addition to the above mentioned safety locking member working by inertia, there is also suggested a device having a safety locking member which is entrained by a pendular mass acting when the deflection is at least 30.degree. with respect to the vertical defined by the axis of the pendular mass.
Finally and in addition to the above device provided with an inertia safety locking member with a fly weight, there is provided a position memory member which allows the use of these seats in two-door vehicles by enabling, in a rapid movement, the disengagement of a front seat when the rear passengers enter or leave the vehicle.